Sounds are transmitted through the outer ear to the eardrum which moves the bones of the middle ear and excites the cochlea. The cochlea is a long narrow duct wound spirally about its axis for approximately two and a half turns. The fluid filled cochlea transmit waves in response to received sounds and in cooperation with the cochlear duct, function as a transducer to generate electric impulses which are transmitted to the cochlear nerves and thence to the brain.
In people with total sensorineural hearing loss, the cochlea does not respond to sound waves to generate electrical signals for transmission to the cochleal nerves. An auditory prosthesis for the deaf therefore requires a suitable stimulation electrode capable of stimulating the auditory nerves. A design of an implantable hearing prosthesis that is currently available for use in patients includes a transmitter, a receiver and an external battery such that the receiver interacts with electrodes placed surgically in the cochlea (Hochmair et al., U.S. Pat. No. 4,284,856 and 4,357,497) so as to selectively stimulate the wall of the cochlea in accordance with the frequency response thereof. The electrodes are typically contained in an electrode carrier that is circular in cross-section and made of a flexible material but is of sufficient stiffness to be guided into the cochlea in the desired coiled shape (Hochmair-Desoyer et al., Annals of the New York Academy of Sciences 405:173-182 (1991)).
In 1980, Hochmair-Desoyer et al. (IEEE Transactions on Biomedical Engineering 27:44) described a basic electrode design that remains the accepted format. A flexible eight channel scala typani electrode carrier was developed which was circular in cross section and tapered slightly to the tip, having a diameter at each contact site along its length that was slightly smaller then the smallest diameter observed at the corresponding length in human scala tympanies. The electrode carrier contained 16 Teflon-insulated 90% Pt-10% Ir wires with a diameter of 1 mil embedded in a silastic body. The contact members were arranged in two rows on opposite sides of the electrode carrier. In the original design, each wire terminated in a ball having a diameter of 0.35 mm which protruded just slightly from the electrode carrier so as to form the contact member. This protruded form of the contact member was later described by Loeb et al. (1983), Med. and Biol. Eng. and Computing 21:241. This design of electrode carrier was capable of being placed up to 22 mm into the cochlea.
An alternative placement of the contact members is within wells on the surface of the electrode carriers where the contact member may be surrounded at the base of the well by an annulus of conducting material. Lim (1987), Abstracts of the Tenth Midwinter Research Meeting of the Association for Research in Otolaryngology, No 66.; Fardeau et al. (1986), EP 0183605 and Stypulkowski (U.S. Pat. No. 4,961,434 and 5,037,497). The electrode carrier bearing the contact members in wells was described as capable of insertion to a depth of 22 mm into the cochlea. One of the problems with the latter design is that the shape of the well permitted the trapping of air bubbles during insertion, this having the effect of interfering with the transmission of signal between the contact and the targeted auditory nerve.
Yet a third approach to contact member placement was described by Clarke in the J. Laryngology and Otology 93:107-109 (1979), where contact members were formed from 0.3 mm wide rings of platinum encircling the electrode carrier. The prosthesis described by Clarke was inserted up to 20 mm into the cochlea but insertion was restricted by the rigidity of the device that resulted from the spaced platinum collars along the length of the electrode carrier. Furthermore, only a minor portion of the electrical current released at the contact site could reach an auditory nerve because the current would be released over an arc of 360.degree. to achieve contact with a neuron positioned at one site only in the arc. Consequently, the device of Clark was relatively energy inefficient.
In summary, existing implantable hearing prosthetic devices have limitations such as those listed below which it would be desirable to overcome so as to obtain a device for insertion in the cochlea that is safe, pain-free and cost effective. These limiting factors, include;
(a) The distance into the cochlea that existing prosthetic devices can penetrate without damaging the basilar partition or the bony spiral lamina. The cochlea is 34 mm in length with the auditory nerves arranged in contact with the cochleal wall in such a way as to capture low pitch sounds at the opening of the canal and high pitch sounds at the far end of the canal. Existing electrode carriers are unable to penetrate greater than about 22 mm into the cochlea. The limiting factors include the friction of the electrode carrier against the cochlea and the overall flexibility versus rigidity of the electrode carrier. The limitations of penetration inherent in existing electrode structures prevent the optimal exploitation of auditory nerves arrayed along the length of the cochlea, necessary for faithful reproduction of a sound. There is a need therefore to develop an electrode carrier capable of insertion along the entire length of the cochlea canal.
(b) The ability to accommodate the variable structure of the cochlea of individual patients in the manufacture of the device. Different patients have cochlea that are foreshortened due to various amounts of ossification of the canals. As a result, a standard electrode carrier is not optimal for all patients. There is therefore a need for a flexible system of manufacture that permits an electrode carrier of optimal length for an individual patient to be made in which the contacts are evenly spaced along the length of the electrode carrier in such a manner as to fully utilize the auditory nerves that are accessible through the length of the cochlea.
(c) The geometry of the placement of the contact members relative to the auditory nerves. An electrode carrier in the cochlea should be of a geometry enabling the surgeon to place contacts as near to the excitable nerve structures as possible. Furthermore, neurons are located on one side of the cochlea only. In the electrode carriers of the prior art which are cylindrical, there is no suggestion or teaching regarding how to localize the position of the contact members in the cochlea so as to minimize the current necessary to provide for the efficient delivery of electrical stimuli to the neurons. There is a need therefore to develop an electrode carrier having contact members that can be located more closely to the neurons.
(d) The accurate determination of the functioning of inserted devices. Back telemetry is used to measure electrode impedances in installed electrodes. When air bubbles are formed, during insertion of the prosthetic device, impedance measurements are abnormal. With existing devices, it is not possible to determine whether an incorrect electrode impedance is due to an air bubble or to a malfunctioning contact. Incorrect impedance readings result in removal of the tested electrode carrier and possible discarding of the device and the repeat implantation of a second electrode carrier.